In dense wavelength division multiplex (DWDM) systems, a pilot tone is used to conduct optical performance monitoring. The pilot tone is a small and low-frequency modulation (e.g. kHz to MHz) applied to a high-speed optical channel, thus providing an in-band ancillary channel for performance monitoring. The pilot tone may be used to carry wavelength and other link-characterizing information. As depicted in FIG. 1, an optical network may have a plurality of nodes, each node including a reconfigurable optical add-drop multiplexer (ROADM). The network may also include a plurality of pilot tone detectors at various locations in the optical network to monitor channel information, such as wavelength, power, modulation format, baud rate, and/or other performance characteristics. Each pilot tone detector (PTD) typically includes a low-speed photodiode and a digital signal processor (DSP).
However, there exist strong discrete tones (“interference tones”) in the high-speed data channel due to factors such as forward error correction (FEC) and digital signal processor (DSP) frame structure, especially when the payload contains an Alarm Indication Signal (AIS), an Open Connection Indication (OCI), or a locked status (LCK). These interference tones may coincide with the pilot tone frequency, making pilot tone detection very challenging.
FIGS. 2 and 3 are graphs plotting the spectra of an AIS payload and a pseudo random bit sequence (PRBS) payload, respectively. Discrete tones depend on the payload, transmitter baud rate, etc. In practice, it may be difficult, particularly for signals carrying the AIS, OCI, LCK payloads, to avoid these tones by selecting non-interfering pilot tone (PT) frequencies because there are so many different baud rates (including clock drift), FEC codes, DSP frame structures, etc.
An improved optical channel monitoring technique would be highly desirable to provide a low-cost, efficient solution for monitoring the performance of optical networks.